You can use the slidechanged
event to get what you are looking for:
Reveal.addEventListener( 'slidechanged', function( event ) {
var currentSection = $('section.present');
var nextId = currentSection.next('section').attr('id');
$('.navigate-left').text($(event.previousSlide).attr('id'));
$('.navigate-right').text(nextId);
} );
Working Plunkr here: http://jsfiddle.net/3fJqZ/109/